1. Field of the Invention
This invention relates to a semiconductive resin composition which, when used as a coating on an insulating layer in a power cable, exhibits improved peelability from the insulating layer, and to a process for producing the same.
2. Background Art
In the field of power cables, there have conventionally been known cables of such a type that semiconductive layers are provided as the internal and external layers of an insulating layer for the purpose of decreasing the electrical field. It is necessary that these semiconductive layers be closely adhered to or bonded to the insulating layer so as to prevent the occurrence of corona discharge. However, when the external semiconductive layer and the insulating layer are excessively adhered to each other, it is extremely difficult to peel the external semiconductive layer from the insulating layer when, for example, two cables of this type are connected with each other. As a result, it takes long time to peel the external semiconductive layer from the insulating layer, and, in addition, the insulating layer tends to be damaged. The peeling operations thus require a considerable amount of time and labor, and a great deal of skill.
Semiconductive layers comprising as base resins ethylene-vinyl ester copolymers, which have been considered to be the most excellent semiconductive layers for use in the cables of this type, have the property of very strongly adhering to olefin polymers used for forming the insulating layer. It is therefore very difficult to peel the outer semiconductive layer from the insulating layer.
An object of the present invention is to provide a semiconductive resin composition suitable for use as a semiconductive layer, which adheres to an insulating layer with a sufficient strength but can be peeled very easily from the insulating layer when necessary and which has mechanical strength good enough for not easily being cut during peeling operation.
In the prior art, the following have been proposed as materials for semiconductive layers:
(1) those materials which are obtained by blending ethylene-vinyl ester copolymers such as ethylene-vinyl acetate copolymers (EVA) having high vinyl acetate contents or ethylene-ethyl acrylate copolymers, or ethylene-acrylic ester copolymers with carbon black; PA1 (2) those materials which are obtained by adding carbon black to halogen-containing resins such as chlorinated polyethylene, chlorosulfonated polyethylene or EVA-vinyl chloride graft copolymers, or to mixtures of these halogen-containing resins and olefin polymers; and PA1 (3) those materials which are obtained by adding carbon black to blends of olefin polymers with polystyrenes, styrene copolymers, butadiene-acrylonitrile copolymers, polyesters or the like. PA1 1) the resin composition, when used as a semiconductive coating layer on an insulating layer in a power cable, can adhere to the insulating layer with a sufficient strength; PA1 2) the semiconductive layer can be easily peeled from the insulating layer, when necessary; PA1 3) the semiconductive layer has good mechanical strength, and hardly cuts when peeled from the insulating layer; PA1 4) carbon black can be thoroughly dispersed in the resin composition; PA1 5) the resin composition has excellent extrusion molding properties; PA1 6) the resin composition is excellent in thermal resistance, and does not emit corrosive gasses; and PA1 7) the resin composition can be crosslinked by a simple process with high productivity. PA1 (A) 5 to 100 parts by weight of a modified ethylene copolymer obtainable by subjecting an ethylene copolymer (a1) and a vinyl monomer (a2) to graft polymerization conditions, PA1 (B) 0.5 to 15 parts by weight of an unsaturated silane compound, PA1 (D) 10 to 110 parts by weight of carbon black, and PA1 (E) 0 to 95 parts by weight of an ethylene copolymer, provided that the amounts of the components shown above are based on 100 parts by weight in total of the components (A) and (E), PA1 wherein the component (B) is incorporated into the composition by subjecting the component (B) to melt graft reaction together with the component (A) and/or component (E) in the presence of 0.01 to 2 parts by weight of a radical generator (C), PA1 the vinyl monomer (a2) unit is contained in the composition in an amount of 5 to 35% by weight of the total amount of the components (A) and (E), and PA1 the degree of crosslinking of the composition is from 30 to 90% by weight. PA1 (A) 5 to 100 parts by weight of a modified ethylene copolymer obtainable by subjecting an ethylene copolymer (a1) and a vinyl monomer (a2) to graft polymerization conditions, PA1 (B) 0.5 to 15 parts by weight of an unsaturated silane compound, PA1 (D) 10 to 110 parts by weight of carbon black, and PA1 (E) 0 to 95 parts by weight of an ethylene copolymer, provided that the amounts of the components shown above are based on 100 parts by weight in total of the components (A) and (E), PA1 wherein the process comprises the step of subjecting the component (B) to melt graft reaction together with the component (A) and/or component (E) in the presence of 0.01 to 2 parts by weight of a radical generator (C). PA1 RSiR'.sub.n Y.sub.3-n PA1 (A) 5 to 100 parts by weight of the modified ethylene copolymer obtainable by subjecting the ethylene copolymer (a1) and the vinyl monomer (a2) to graft polymerization conditions, PA1 (B) 0.5 to 15 parts by weight of the unsaturated silane compound, PA1 (C) 10 to 110 parts by weight of the carbon black, and PA1 (D) 0 to 95 parts by weight of the ethylene copolymer, provided that the amounts of the components shown above are based on 100 parts by weight in total of the components (A) and (E), PA1 wherein the component (B) is incorporated into the composition by subjecting the component (B) to melt graft reaction together with the component (A) and/or component (E) and 0.01 to 2 parts by weight of the radical generator (C); the vinyl monomer (a2) unit is contained in the composition in an amount of 5 to 35% by weight of the total amount of the components (A) and (E); and the degree of crosslinking of the composition is from 30 to 90% by weight.
However, the above-described conventional semiconductive materials have the following drawbacks.
The materials (1) are, as mentioned previously, poor in the peelability from the insulating layer.
The materials (2) possess improved peelability from the insulating layer. However, the halogen-containing resins generate and emit corrosive gasses when thermally decomposed at high temperatures, and the gasses promote the corrosion of production apparatuses, or corrode copper shield tapes used for cables.
The materials (3) also show improved peelability from the insulating layer. However, they are poor in the compatibility between the olefin polymers and the other resins. Moreover, in order to attain sufficient peelability from the insulating layer, the amount of the resins to be blended with the olefin polymers should necessarily be large. Semiconductive layers made from such materials are considerably brittle, and thus undesirably cut during the peeling operations.
For use of the above-described materials (1), (2) and (3) as coating layers for power cables, the following two-step preparation process has been employed as so to impart thermal resistance to the materials: organic peroxides are added to the materials and the mixtures are molded at low temperatures; and the molded products are cross-linked by using a specific crosslinking apparatus.
On the other hand, as a method for attaining drastically increased productivity as compared with that attained by the above crosslinking method using organic peroxides (hereinafter referred to as peroxide crosslinking method), there has been proposed the silane crosslinking method. The silane crosslinking method is such that, after silane-containing polyolefins as described in Japanese Patent Publications No. 1711/1973 and No. 23777/1987, etc. are subjected to molding, the molded products are crosslinked in the presence of silanol condensation catalysts in an aqueous atmosphere. This silane crosslinking method is advantageous over the peroxide crosslinking method in that the crosslinking apparatus for use in this method is simpler than that for use in the conventional method and that the productivity attained by this method is much higher than that attained by the conventional method. For this reason, the use of a silane-crosslinked polyethylene for the insulating layers of low-voltage cables has been spread widely. Moreover, it has been proposed to apply a silane-crosslinked polyethylene also to semiconductive coatings in high-voltage cables, as disclosed in Japanese Patent Publication No. 31947/1995 and Japanese Patent Laid-Open Publication No. 293945/1992. However, the conventional silane-crosslinked polyethylene is still unsatisfactory in the peelability from the insulating layer of a power cable.